In recent years, a thin and light display device such as a liquid crystal display device has been widely used as a display for a personal computer and a display for a small-size television. In particular, when a liquid crystal display device is used for portable devices such as a portable computer and a portable television, it is required to reduce power consumption, because power is supplied by a battery.
A liquid crystal display device of transmissive type is provided with a backlight at a back face of a liquid crystal panel. Here, reducing power consumption of the backlight allows a reduction in power consumption of the liquid crystal display device.